X-rays are commonly used to diagnose medical conditions in humans and animals and in many industrial uses. For medical purposes, an object, typically a portion of the body of a person or animal, is exposed to X-rays while positioned in proximity of a radiation sensitive media. A latent image of the object is formed in the radiation sensitive media which can be subsequently developed to aid the medical practitioner in viewing aspects of the body which can not be directly visually seen. Conventionally, silver halide films in conjunction with a light emitting phosphor sensitive to X-rays are used for this purpose.
More recently, radiation detectors have been constructed which eliminate the use of silver halide films in the detection of the object subjected to X-ray radiation. Radiation detectors are designed to detect incident radiation and convert that incident radiation into an electrical signal which can be utilized to construct a pixel by pixel representation of the image. A radiation detector is sensitive to X-rays but instead of forming an image of the object in silver halide film, the radiation detector directly converts the energy contained in the X-rays to electrical signals. These electrical signals can then be digitized, if they aren't already digital, and can digitally represent the image of the exposed object in a pixel by pixel basis. The digital representation of the image can then be viewed on conventional displays or monitors, transmitted to remote sites, stored electronically and printed in imagers to provide a visual output similar to that which medical practitioner is accustomed.
Such radiation detectors typically involve an array of photosensitive detector elements constructed in array to form an entire X-ray image. Typical X-ray images are 14 inches (35.6 centimeters) by 17 inches (43.2 centimeters) and typically comprise an array of pixels of approximately 4,000.times.5,000. Thus, as many as 20,000,000 pixels may be contained in a single image.
Since direct digital radiation detectors are formed with an individual photosensitive element for each pixel of image, the same number or a greater number of individual photosensitive elements are required. Such photosensitive elements are typically formed using integrated circuit technology. Not only are such difficult to construct in a size useful for radiology the yield of such devices decreases rapidly as the size increases.
Thus, a tiling approach has been used to construct large radiation detectors. Using this approach, a plurality, typically a large number, of individual radiation sensitive tiles are constructed, typically using integrated circuit technology. The tiles are relatively small and, thus, relatively easy to manufacture using conventional techniques. The individual tiles are then placed adjacent each to form a large radiation detector suitable for use with conventional X-ray machines.
The tiling approach, however, has many problems. Since each individual tile has a surface onto which X-ray radiation is incident and since only a part of that surface of actually sensitive to X-ray radiation, the overall radiation detector has many "dead spots" or areas which are insensitive to the X-ray radiation.
The tiling approach has been widely studied, especially in the area of charge coupled devices (CCD's). In U.S. Pat. No. 4,467,342, thinned imager chips are arranged end-to-end and accurately positioned relative to one another so that the proper spacing period between adjacent imager pixel detectors is maintained. However, the chips are put together by using a lap joint (shingling) rather than a butt joint. The lap joint method creates a non-uniform surface which in turn translates into a non-uniform coating of the phosphor covering the chips. This non-uniformity can cause scattering of the X-rays leading to loss in resolution and also to loss of information at the tile overlap interface due to insufficient coverage of the phosphor.
In U.S. Pat. No. 4,810,881, a method of making a large area X-ray detector is described which comprises several detector chips placed end-to-end. Each chip has its own addressing and reading circuits. The addressing circuit is located on an edge of the insulating substrate that bears the detectors and the reading circuit is located on the opposite side of the substrate from the detectors. In this method each tile must allow sufficient space between the tiles for column connections thus leading to dead space resulting in loss of information.
In U.S. Pat. No. 5,105,087, the assembly of a large area X-ray sensor which is formed from a plurality of smaller solid state sensors is described. The large area sensor includes at least a first solid state sensor having an X-ray detector region and a blind non-detector border region. Positioned adjacent to the first sensor is a second solid state sensor having an X-ray detector region and a blind non-detector border region with respective non-detector regions being contiguous. A third solid state sensor having an X-ray detector region is positioned to overlie the first and second solid state sensors in such a way that the X-ray detector region of the third sensor overlies the blind non-detector regions of the first and second sensors, however, the third sensor also incorporates a blind non-detecting region. In this method the alignment of the third solid state sensor is very critical and difficult to accomplish. The placement of the third solid state sensor on top of the first and second sensors results in a non-uniform surface. This non-uniformity can cause scattering of the X-rays leading to loss in resolution.
In a typical sensor chip, described in the previously mentioned patents, the radiation sensitive area is contained at the center and is surrounded by the radiation non-sensitive area which is utilized for the addressing, read-out and metal contacts, thus leading to inevitable loss of information in the conventional large area sensors.